Breast cancer commonly metastasizes to the skeleton in patients with advanced disease to cause bone destruction (osteolytic metastases) and the associated pain, hypercalcemia, fracture and nerve compression syndromes. Histological analysis of osteolytic bone metastases indicate that the bone destruction is mediated by the osteoclast. Tumor-produced parathyroid hormone-related protein (PTHrP), originally identified as a hypercalcemic factor and a known stimulator of osteoclastic bone resorption, is a major mediator of the osteolytic process. Transforming growth factor (TGF) beta, abundant in bone matrix and released as a consequence of osteoclastic bone resorption, stimulates PTHrP production by cancer cells. Over the past five years, work supported by this grant has identified a new role for TGFbeta in malignancy to promote breast cancer osteolysis by stimulating tumor production of PTHrP. This notion is contrary to the popular belief that the major role of TGF beta in malignancy is that of a tumor suppressor. This competitive renewal will dissect the molecular mechanisms responsible for the regulation of breast cancer PTHrP production by TGFbeta. Preliminary data from the Principal Investigator's laboratory indicate that 1) the effect of TGF beta to stimulate PTHrP production is mediated by both the Smad and mitogen-activated protein (MAP) kinase signaling pathways; 2) A constitutively active estrogen receptor (ER)- alpha mutation (Tyr537Asn), identified from a human bone metastasis, when expressed in human breast cancer cells further increased TGF beta-stimulated production of PTHrP. The following specific hypotheses will be tested: 1. TGFf3 promotes osteolytic bone metastases by breast cancer via both Smad and MAP kinase signaling pathways to increase PTHrP. 2. A constitutively active ER-a increases PTHrP production and osteolytic metastases by breast cancer cells through interaction with the TGFb signaling pathways. 3. Blockade of the TGFb signaling pathway in the receptor level will be a more effective way to decrease osteolytic bone metastases than targeting either the Smad or MAP kinase pathways alone. 4. TGFb-responsive factors other than PTHrP contribute to breast cancer Osteolysis. The following specific aims are proposed to test the hypotheses: 1 )To determine the mechanisms by which TGFb increases PTHrP production by breast cancer cells. 2) To determine if neutralization of TGFb will effectively block osteolytic metastases How does this compare with neutralization of PTHrP or inhibition of steoclastic bone resorption? 3) To determine if other TGFb-responsjye factors, recognized and unrecognized, have a role in osteolytic metastases and if such factors enhance the effects of PTHrP on osteolysis. The proposed work will provide insight into the mechanisms of osteolytic bone metastases which will result in effective therapy.